A system of mutants at three distinct loci is being studied in order to investigate the regulation of gene expression in Drosophila. All mutants at these three loci are deficient for three enzymatic activities, xanthine dehydrogenase, aldehyde oxidase and pyridoxal oxidase. Genetic and biochemical evidence from this laboratory as well as from other sources argues that the three loci do not provide any major structural elements to the enzymes they affect. One aspect of the present work utilizes a combination of chemical selective agents to screen for newly induced mutations at two of the three loci. New mutants will provide the raw material for determining the fine structural organization of the loci. An array of mutants is also being sought to facilitate the biochemical analysis of the function of each gene. Recent biochemical evidence we've obtained indicates that two of the loci exert their effects on the expression of the enzymatic activities via the production and/or utilization of a molybdenum cofactor. We intend to investigate the biochemical relationship between the three "regulatory" genes and the cofactor in order to understand the role of this cofactor in affecting the final expression of three disparate enzymatic activities.